This invention relates to amplifiers and, in particular, to a mute circuit for an audio amplifier.
Audio amplifiers are utilized to amplify signals in the audio frequency range wherein the amplified signal is used to drive a load such as a speaker.
A basic audio amplifier typically includes an operational amplifier having inverting and non-inverting inputs and an output. The non-inverting input of the operational amplifier is coupled to receive a reference voltage. The inverting input of the operational amplifier is coupled through a resistor and typically a coupling capacitor to receive an input voltage. The output of the operational amplifier is typically coupled through a coupling capacitor to provide an output voltage. Further, the output of the operational amplifier is coupled back to the inverting input of the operational amplifier through a feedback resistor. Further, there may also be a filter capacitor coupled across the feedback resistor.
There exists many situations where it is desired to mute an audio amplifier. In a mute mode, the output signal of the amplifier is heavily attenuated with respect to the input signal.
One attempt that prior art has made for muting an audio amplifier is to couple the current carrying electrodes of a transistor across the inverting input and the output of the operational amplifier. Thus, when a logic high voltage level is applied to the control electrode of the transistor, the transistor is driven into saturation thereby effectively shorting the inverting input of the operational amplifier to the output of the operational amplifier. However, in order to receive acceptable attenuation, the saturation resistance of the transistor must be much less than the feedback transistor of the operational amplifier thereby requiring a large device. Further, when muting is switched on, a DC shift in voltage will occur at the output of the operational amplifier thereby causing a pop to a speaker that is typically coupled to the output of the operational amplifier.
Another attempt that prior art has made for muting an audio amplifier is to couple a pair of transistors between the output of the operational amplifier and the coupling capacitor. In this implementation, when a logic high voltage is applied to the bases of the transistors, the transistors are driven into saturation allowing the input signal to be passed through the audio amplifier to allow normal operation. On the other hand, when a logic low voltage level is applied to the bases of the transistors, the transistors effectively provide an open circuit thereby preventing the input signal from reaching the output. However, in this implementation, when the transistors are turned on, they must have a very low saturation resistance to avoid attenuation of the output signal, for example, less than 15 ohms. This is typically difficult to achieve on an integrated circuit without substantially increasing the cost of the integrated circuit.
Another disadvantage of the second attempt is that a non-linear device is coupled in series with the load thereby causing distortion.
Hence, a need exists for an improved mute circuit for an audio amplifier.